Recently replaced alternator and battery (was told by Brakes Plus that was the issue). The car (with no warning) sometimes just won’t start. Wether after driving it or if it’s been sitting for a day or two, sometimes it will only turn over once, but never start. Not sure if it’s a timing issue or an issue with spark/fuel. There’s also a loud click in time with engine rotation when it does start, may be an injector, also not sure with that.
You need to remove a plug and lay it against the frame. Have someone turn the key and see if you have spark. If you have spark, then fuel pressure will need to be checked.
A 28 year old Audi has many places to look - you’ll need to check all wires in the starting system, all connections, all switches. Check voltages at each spot as you try to start the car. VW/Audis of this age often have weak grounds, I added a ground directly from my battery negative to the block.
If the vehicle has an automatic transmission, one thing that can cause your problem is a cracked flex plate.
The flex plate has a ring gear that the starter gear engages. If the flex plate is cracked, it can cause the ring gear to become misaligned with the starter gear. This can then cause the starter to bind when it gets to the area of the crack in the flex plate. Then when the starter is able to turn the engine over, you hear a loud clicking sound each time cracked area of the flex plate comes around where the ring gear comes in contact with the starter gear.
I’ve seen this happen on a few vehicles. And Ive seen it stump a few mechanics.
Maybe you could clarify your post a little. Do you hear that loud click all the time the engine is running, or only during cranking? And explain more what you mean by it “turns over once”. Do you mean it makes that rrr rrr rrrr engine cranking sound but only for a half second then it stops making that sound, and doesn’t start and run, even if you keep the key in “start” position?